Deregulation of c-Myc expression is implicated in pathogenesis of many human cancers including breast cancer. Estrogen receptor alpha can increase the rate of c-Myc transcription through the recruitment of a variety of cofactors to the c-Myc promoter, yet the precise roles of these cofactors in transcription and tumorigenesis are largely unknown. We have identified a putative tumor suppressor, TIP30, also called CC3, that can specifically regulate ER alpha-mediated c-Myc transcription. Our objective is to investigate the role of TIP30 in the regulation of transcription of ER alpha-target genes and tumorigenesis. The specific aims of this proposal are to: 1) investigate whether deletion of TIP30 increases expression of ER alpha-target genes in the epithelial cells of the mammary glands in vivo. 2) investigate whether breast cancers from patients harbor mutations in the TIP30 gene. 3) evaluate the susceptibility of TIP30-mutant mice to mammary tumorigenesis. These studies will have a significant impact on three important aspects. They will provide: 1) a molecular basis for a TIP30-mediated regulatory pathway in mammary gland development, 2) direct evidence for the role of a transcription cofactor in breast carcinogenesis, and 3) a system for studying biologically relevant factors and events responsible for the pathogenesis of breast cancers. Therefore, this proposal will not only yield insights into the mechanisms for regulation of gene expression intumorigenesis but also provide a potential target for the diagnosis and treatment of human cancers.